In the hydrocarbon exploration and production industry, a milling tool, which is commonly referred to as a section mill, may be used to remove a length of metal tubing within a borehole, by comminuting the tubing to swarf. Merely by way of example, one circumstance in which milling is used is when removing tubing/casing and exposing the rock formation around a borehole in preparation for setting a cement plug to meet regulatory requirements when a well is abandoned.
A conventional section mill, such as the K-Master section mill supplied by Schlumberger, is incorporated into a drillstring which extends up to the surface and also extends forwardly beyond the section mill itself. The portion of the drill string which extends forwardly from the section mill includes components which are dimensioned to have an exterior diameter slightly smaller than the interior diameter of the tubing which is to be removed. These components locate the axis of the drillstring relative to the axis of the tubing which is to be milled.
In many instances, the cutting blades of a conventional section mill can be expanded outwards relative to the body of the tool. The ability to expand allows the mill to be inserted along the tubing to a desired starting point and then to cut outwards through the tubing before being advanced axially to extend the length of tubing which is being removed.